1. Field of the Invention
The present invention relates to strain gage load cells, and more particularly to bending beam load cells with means for eliminating errors caused by off center loading on the bending beam load cells.
2. Description of the Related Art
Bending beam load cells have been extensively used for weighing since about 1950. A common design is the Single Point Load Cell, which comprises two parallel bending beams on top of each other, separated by rigid end blocks. The load cell is cantilevered from a base supporting one end block, and the other end block supporting a load platform. Axially oriented strain gages are bonded to each beam near the joints with the end blocks.
When a load acts on the load platform, the rigid end pieces force the two bending beams to flex into flat S-shapes. One strain gage on each bending beam will accordingly sense tension, while the second strain gage will sense compression. The four strain gages are connected in a bridge circuit to provide an output signal used to measure the load.
If the symmetry of a single point load cell were perfect, the output signal from the bridge circuit would be a true measure of the vertical component of the load, independent of the position and direction of the load on the load platform. In practice, there are small differences in the strain sensitivity of the four strain gages, and small mechanical differences in the bending beams, as well as errors in the position of the gages on the bending beams. These errors upset the symmetry of the load cell, so there will be errors in the output signal caused by movement of the load application point, both along the load cell axis (“Front-to-back off center load error”), and crosswise to the load cell axis (“Side-to-side off center load error”). The latter off center load error is also known as torque sensitivity, because side to side off center loading subjects the load cell to torque.
From the earliest use of single point load cells it has been known that both of these errors can be minimized by careful filing of the bending beams near the strain gages. The filing requires great skill, and it is time consuming, but it is still the most used method for compensation of off center load errors in single point load cells.
A serious disadvantage with filing of the bending beams to compensate for torque sensitivity is that it makes it impossible to seal the load cell. A sealing bellows can be made soft in the bending direction, but it will always be stiff against torque, so it will have a substantial effect on the torque sensitivity of the sealed load cell. Filing, however, can not be done after the bellows is in place.
U.S. Pat. No. 3,576,128 to Lockery describes an effective method for electrical compensation of the front-to-back off center load error, but it is not applicable to compensation of torque sensitivity.
U.S. Pat. No. 4,453,609 to Griffin et al describes methods for electrical compensation of side-to-side off center load error for a single point load cell. The Griffin methods require either two pairs of axially oriented strain gages placed on opposite sides of the center line of each bending beam, or alternatively two strain gages mounted on the centerline of each bending beam, but with off-axis orientations. The compensation procedure described in the referenced patent is very complicated. It has not been sucessfully used commercially.
U.S. Pat. No. 5,610,343 to Eger et al describes methods for electrical compensation of both types of off center load errors in a bending beam load cell with multiple axially oriented strain gages on each bending beam. Microprocessor sampling of different pairs of strain gage signals in a bridge circuit is used to calculate off center loading errors, and error corrected signals are calculated at frequent intervals. This method is quite complicated, and requires nonstandard electronic instruments for display of load signals. It has not been successfully used commercially.